1. Field of the Invention
This invention relates to methods of measuring physical properties of highly spin-polarized (half-metallic) ferromagnetic materials and superconductors.
2. Description of the Prior Art
Spin polarized transport effects in materials is an important and rapidly developing area of basic research and technology. This new field, known as magnetoelectronics, is spurring the development of new devices that cannot be realized with existing charge-based semiconductor electronics. A central component of these devices is ferromagnetic materials which are ideally 100% spin polarized, in which the conduction electrons have only one spin state available at the Fermi energy. Recent point contact experiments have indicated that the spin polarization in chromium dioxide (CrO2) approaches 100%, as disclosed in an article by R. J. Soulen et al., Science 282, 85 (1998). Ultra-thin layers of highly spin-polarized CrO2 have potential applications in giant magnetoresistance (GMR) devices.
According to Osofsky et al., Physica C341-348 (2000) 1527-1530, these new classes of electronic devices have properties that are determined by the electron spin. The performance of these devices is enhanced as the spin polarization, P, of the ferromagnetic components increase. A ferromagnetic metal has excess electrons with one spin orientation and is characterized by majority and minority spin sub-bands. The total magnetic moment of a ferromagnetic metal is the difference of the moments of the electrons in the majority and minority sub-bands. Transport current is the sum of currents from majority and minority conduction bands. A spin polarized current results when the conductivities of two conduction bands are not equal.
Soulen et al. have shown that spin polarization P can be determined by using point contact conductance measurements with a superconducting tip and interpreting the results using the Blonder-Tinkham-Klapwijk (BTK) model of Andreev reflection at a normal/superconductor interface. This is generalized to include a spin polarized normal metal. Other demonstrations of similar techniques have also been developed, specifically for ferromagnetic materials by microlithography forming F/S point contacts.
In a conventional Andreev reflection process for a superconductor/unpolarized normal metal junction at T=0, where an electron approaches the S/N interface, a Cooper pair can enter as long as charge and spin are conserved. Conventional Andreev-reflection is characterized by a doubling of the normalized conductance, G(V)/GN, below Δ/e where GN is the conductance for V>>Δ/e. This is the result of a hole conductance channel opening up in N thus doubling the number of N carriers. Since the process is blocked for the 100% spin polarized N material, no current can flow, i.e. the conductance goes to zero for voltages below Δ/e. Simple BTK analysis assumes ballistic contacts, spherical Fermi surfaces and particle-hole momentum conservation. Experiments have explored more realistic theoretical treatments.
Magnetic oxide thin films are used in applications including, but not limited to, media for magnetic recording, and in spintronic devices. Also, ferromagnetic oxide material can be used as memory storage material as part of a magnetic multilayer to store information in computer and hard drives. Fully spin-polarized material, called a “half-metal,” behaves as an insulator for the transport of a first electron spin orientation and as a conductor for a second spin orientation.
In point contact Andreev-reflection (PCAR), a metallic point contact is formed between the spin polarized magnetic sample and a superconducting tip. Spin polarization of the sample is determined by the electronic transport properties at the point-contact junction between the tip and the sample.
There is a currently a great deal of research in a new field called “spintronics,” where the manipulation of the spin of an electron could be incorporated into an electronic device. This would have the advantages of decreased power consumption, non-volatility, increased data processing speed, and increased integration densities compared to conventional semiconductor devices. One important aspect of spintronics is the fundamental research into the spin polarization value associated with any new material.
The experiments that have been conducted in this field of research do not have the range of magnetic field and temperature control that is required for certain half-metals and superconductive tips. What is needed is a reliable system of measuring spin polarization with both coarse and fine control and variability of magnetic field and temperature.
However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in this field how the needed system could be provided.